Fuel cells are being used to power systems such as vehicles and stationary power plants. A fuel cell power module (FCPM) is often used to control power (or current) of the fuel cell. During operation, the system requests various power (or current) levels from the FCPM to satisfy a load condition. For example, a load condition in a vehicle increases when the driver depresses an accelerator and decreases when the vehicle is traveling on a downhill grade.
The system preferably knows the available power (or current) output of the FCPM. If the load is too low relative to supplied fuel and oxidant, the FCPM passes too much fuel out of the stack, which is inefficient and may damage downstream components. If the load is too high relative to the supplied fuel and oxidant, the fuel cell stack may be damaged.
Designing a closed loop control system for a fuel cell requires knowledge of both system loads (such as vehicle loads) and the fuel cell. A significant amount of calibration and tuning of the closed loop control system is typically required to obtain efficient operation. When inevitable design changes are made to the system or to the fuel cell, the closed loop control system must be recalibrated and retuned. This recalibration requirement reduces the flexibility of the fuel cell system, makes the fuel cell system less modular, and may require both the system and the fuel cell to be designed together.
One conventional control approach commands fuel and oxidant to the fuel cell and varies the load to accept the output of the fuel cell. This control approach severely restricts the architecture and operation of the system incorporating the fuel cell. For example, this approach was found to be unacceptable for vehicles. For consumer acceptance of fuel cell vehicles, fuel cells must respond more quickly and accurately to driver input. This control approach is better suited to components and subsystems where the load is not as critical.
Another control approach employs a closed loop control system that is based on a desired fuel cell output such as current or power. The fuel and oxidant inputs are modified until a desired output is achieved. Another approach commands fuel (such as H2, natural gas, gasoline, liquid propane, methanol, etc.) and oxidant (such as oxygen or air). The input commands are then adjusted based on a resulting power output.